Mostly because they risk much but have little to gain. Ford is putting a stellar reputation and a glowing career on the line--to say nothing of her family's well-being. There are many ways that this could end very badly for her, and she's obviously been keenly aware of that from the very start.

But as you suggest, determining guilt or innocence at tomorrow's hearing is likely far too lofty a goal, and instead we'll be left trying to measure shreds of "reasonable doubt." I bet the R's will be making ample use of those words after the hearing...and ignoring the fact that they cut both ways: does one entertain doubts about Ford's veracity, or about K's fitness for the supreme court? I just hope they recall the wisdom of the immortal Johnnie Cochran, who said "If in doubt, you've got to toss the lout." (Or something to that effect.)